The present invention relates to material handling machines and is particularly applicable to plotters in which elongated strips of sheet material must be accurately indexed across a plotting table during a series of plotting operations.
Problems associated with accurately indexing elongated strips of limp sheet material, such as paper, over a work surface that is relatively short compared to the length of the strip are well known in the art. U.S. Pat. Nos. 3,844,461 and 4,131,224 describe mechanisms utilized in plotters for accurately indexing the material so that a continuous plot can be produced along a strip of plotting material without discontinuities between sections that are plotted before and after an indexing operation. It is well known that strips of sheet material do not move precisely along a track between a feed roll and a take-up roll on which the strips are wound. The strips tend to shift laterally of the track in an irregular manner unless additional controls over the lateral movements are provided. The positioning problem is controlled in the above-referenced patents by coupling the strip of sheet material directly to the same carriage that carries the plotting tool and then shifting the strip by means of the carriage. In this manner, there is precise correspondence between both the longitudinal and lateral positioning of the plotting instrument and the strip of material at the junction between portions of the plot made before and after the indexing operation.
It has been determined, however, that even with the indexing method described in the above-referenced patents, positioning errors may arise from tension that remains in the strip of material on the table after the indexing operation is complete. For example, when the strip of material is released from the plotting carriage, residual stress or tension in the paper may shift the strip from the position in which the material was placed by the carriage.
It is accordingly a general object of the present invention to provide a method and apparatus for indexing limp sheet material with accuracy and without the errors arising from residual tension and stresses in the material that occur in the prior art.